Filtration system

ABSTRACT

A pressure filtering method and apparatus in which the residual process liquid present in the pressure filter housing at the end of a filtration cycle is discharged from the housing, prior to the backwash cycle, and is sent to a recovery system and is treated so that it can be returned to the main stream of the process liquid.

United States Patent Reece [54] FILTRATION SYSTEM [72] Inventor: ThomasL. Reece, Portage, Mich.

[731 Assignee: Dover Corporation, New York,

[22] Filed: June 1, 1970 [21] App]. No.: 41,986

[52] US. Cl. ..210/82, 210/333, 210/425 51 1111. c1. ..B0ld 29/38 [58]Field of Search ..210/76, 82, 333,425, 426

[56] References Cited UNITED STATES PATENTS 2,301,803 11/1942 Davis..210/82 x 3,029,948 4/1962 McKay (..210/82 X [151 3' 3,692,178 14 1Sept. 19,1972

3,056,499 10/1962 Liddell ..210/82 X 2,828,862 4/1938 Johnson ..210/823,373,104 3/1968 Ryan ..210/82 X 3,436,260 4/1969 Nuff ..210/82 XPrimary Examiner-John Adee Att0mey-Woodhams, Blanchard and Flynn 5 7]ABSTRACT A pressure filtering method and apparatus in which the residualprocess liquid present in the pressure filter housing at the end of afiltration cycle is discharged from the housing, prior to the backwashcycle, and is sent to a recovery system and is treated so that it can bereturned to the main stream of the process liquid.

17 Claims, 6 Drawing Figures PATENTEDSEP 1a 1972 SHEET 2 0F 3 INVENTIORmama 4 a m? ,W/am KI PATENIEDsEP 19 m2 sum 3 or 5 H24 I P FILTRATIONSYSTEM BACKGROUND OF THE INVENTION This invention relates to an improvedfiltering method and apparatus and, more particularly, relates to afiltering method and apparatus and, more particularly, relates to afiltering method. and apparatus in which the process liquidpresent in afiltration system at the end of a filtration cycle is salvaged, forreuse, prior to backwashing the filter.

It is well known to carry out filtration operations using filters inwhich the process liquid to be filtered is supplied under pressure,gravity or pump, into the space between an outer housing and an internalfilter medium. The process liquid then passes through the filter mediumso that solids of a size larger than the openings in the filter mediumare retainedon the filter medium and the resultant filtrate then flowsfrom the filter to the system for utilizing the filtrate. It isnecessary periodically to clean the filter medium to remove thecontaminate build up therefrom. For this purpose, it is known tobackwash the filter by flowing a backwashing liquid through the filterin the reverse direction. The backwashing liquid leaving the filterhousing and the filter cake particles entrained therein are thendischarged, ordinarily to a drain.

As example of a particularly difficult type of filtration operation isthe filtration of heavily pigmented slurries in the paper-makingindustry. Such slurries include clay coating slurries, paper sizingslurries and similar particle loaded liquids. The filtration operationis intended to remove undispersed solids, paper fibers and otherunwanted particles. It has been found that such slurries can be filteredmost effectively by using an in-line pressure filter that is equippedwith an internal vibrator which imparts high frequency low amplitudevibration to the slurry being filtered. These units provide high flowrates per unit of floor space required and there is little or noflooding, foaming and post-filter contamination. In such installations,the individual filter elements or mediums are cleaned in place'bybackwashing.

In the prior art backwashing procedures, the residual process liquid,including the slurry to be filtered and the resultant filtrate, presentin the pressure filter housing and associated piping at the end of thefiltration cycle is not removed prior to the backwashing cycle. Thus,the residual process liquid is merely mixed with the backwash-ing liquidand is discharged therewith to drain. This is undesirable for a numberof reasons. For example, the process liquid in many instances is quitevaluable so that a substantial economic loss occurs when the residualprocess liquid is mixed with the backwashing liquid and is discharged todrain. This is true, for example, of the pigmented slurries forpapermaking uses as described above. Moreover, the residual processliquid in effect contaminates the backwashing liquid, which usually iswater, so that the waste treatment requirements necessary prior todischarging the contaminated backwash liquid to the plant effluentsystem are increased.

An important factor in the design of pressure filtration systems,particularly for handling process liquids with a high solids content,such as paper coating slurries, is that the flow path for the processliquid should be smooth and uninterrupted so as to avoid the presence ofpockets or dead spots where the process liquidmight collect. Because ofthe high solids content and high. viscosity. of the process'liquid,solids may deposit in such pockets or dead spots. Such agglomeratedsolids will become loosened eventually and reenter the process liquid,but since they are of larger size than the solids that flow throughthefilter medium, they may detrimentally. affect the quality of thecoated product. In particular, pockets, dead spots and the like shouldbe avoided downstream of the filter medium. Moreover, the possibility offurther contamination or dilution of, or the presence of air in, thefiltered process liquid should be avoided.

Accordingly, it is an object of this invention to provide an improvedfiltering method and apparatus in which the residual process liquidpresent in a filtration system at the end of the filtration cycle isremoved from the system, prior to backwashing the filter, and isrecovered for reuse.

It is a further object of this inventionto provide an improvedfilteringmethod and apparatus, as aforesaid, in which a separate piping andvalving system is provided to segregate the residual process liquid, tobe recovered, from the backwash liquid.

It is a further object of this invention to provide an improvedfiltering method and apparatus, as aforesaid, in which the recoveredresidual process liquid is subjected to a reconditioning operation sothat it can be returned to the filtration system.

It is a further object of this invention to provide an improvedfiltering method and apparatus, as aforesaid, in which the additionalconduits and valves provided from the recovery operation are locatedoutside of the flow path of the process liquid during the filtrationcycle so as to reduce the possibility of contamination of the processliquid.

Additional objects and purposes of he invention will become apparent topersons acquainted with equipment of this type upon reading thefollowing description and inspecting the accompanying drawings.

BRIEF DESCRIPTION OF'THE DRAWINGS In the drawings:

FIG. 1 is a perspective view of a typical prior art system with whichcan be used the filtering method and apparatus of the present invention;

FIG. 2 is a perspective view of a filtration apparatus according to thepresent invention;

FIG. 3 is a simplified, central sectional view through a filter unit andthe associated conduits used in the filtration apparatus disclosed inFIG. 2;

FIG. 4 a simplified sectional view showing a preferred construction ofthe process liquid recovery and reconditioning system;

FIG. 5 is a sectional view substantially as taken along the line V-V ofFIG. 3; and

FIG. 6 is a simplified sectional view of a modified recovery andreconditioning system.

SUMMARY OF THE INVENTION The invention is used in conjunction with aknown filtration method and apparatus including one or more filter unitseach having a housing and a filter medium disposed in the interior ofthe housing. The filter units have inlets and outlets for (1) during thefiltration cycle, supplying a process liquid to the housing on one sideof the filter medium and discharging the filtered process liquid from.the other side of the filter medium, and (2) during the backwashingcycle, flowing a backwashing liquid in the reverse direction through thehousing and filter medium in order to remove the contaminate build upfrom said filter medium. According to the invention, during the intervalbetween the end of a filtration cycle and the beginning off a backwashcycle, the process liquid present in the filter unit and associatedpiping at the end of the filtration cycle is removed from the filterunit, usually with the assistance of a pressurized gas such as air, andis discharged into a recovery and reconditioning system. Following theremoval of the residual process liquid from the filter unit, thebackwashing liquid is supplied in order to clean the filter medium. Thebackwashing liquid and the entrained solids therein are usuallydischarged to a drain.

DETAILED DESCRIPTION Since the invention can best be understood in thecontext of one typical prior art system with which it i intended tooperate, the description will first be directed toward such a system.

. Referring to the drawings, FIG. 1 illustrates a prior art system whichincludes a pressure filter apparatus of the type which can be modifiedto employ the subject mater of the present invention as describedhereinbelow. It will be understood that the prior art system shown inFIG. 1 is presented, for illustrative purposes only, to indicate atypical installation with which the invention can be used to advantage.It will be further understood that the invention can be used with a widevariety of process systems employing filter units.

The system illustrated in FIG. 1 is intended for applying a coating ontoa paper web 10. The web continuously moves in the direction of the arrowA between an applicator roll 1 1 and a backing roll 12 The lower portionof the periphery of the applicator roll 1 1 dips into a supply .offiltered coating material, i.e., a liquid slurry, in the rightwardportion 14 of the pan 13. The applicator roll 11 rotatescounterclockwise as shown in FIG. 1 so that it picks up coating materialfrom the part 13 and applies it to the underside of the paper web 10. Acoater blade 16 removes excess coating material from the web 10 and saidexcess coating material drops into the leftward portion 17 of the pan13. Suitable means, such as an upright wall 18, separates the portions14 and 17 of the pan from each other so that the fresh filtered coatingmaterial in portion 14 is prevented from mixing with the scraped-offcoating material in the portion 17. The coating material in portion 17flows through a conduit 19 into the supply tank 21 for the coatingmaterial. Filtered make-up coating material is added to the supply tank21 through the conduit 22. Coating material is withdrawn from the tank21 by the pump 23 and is fed through the conduit 24 into the pressurefilter apparatus 26. If desired, a pressure sen- 28 connected inparallel. It will be understood that greater or lesser numbers ofpressure filter units can be provided as needed to meet the requirementsof the particular installation involved.

The coating liquid flows from the conduit 24, through the pressuresensor 27, if used, into a process liquid inlet header 29 and thenceinto the lower ends of the pressure filter units 28. The liquid flowsupwardly through the pressure filter units and is filtered thereby inorder to remove unwanted particles therefrom. The filtrate from thepressure filter units flows into a process liquid outlet header 30 andthence through a conduit 31 into the rightward portion of the pan 13 incondition for application to the web.

The respective pressure filter units 28 can be backwashed at selectedintervals to remove contaminate build up therefrom. For this purpose, aconduit 32 is provided to supply a backwash liquid, such as hot water ora suitable solvent, through a valve 76 to a backwash inlet header 33.The backwash header 33 is connected to the upper ends of the pressurefilter units 28 by means of valves 71 to be described in greater detailhereinbelow. Further, the lower ends of the pressure filter units 28 areconnected by valves 72, also to be described in greater detailhereinbelow, to a backwash outlet header 34 which in turn is connectedby conduit 36 to an effluent drain system.

Referring now to FIG. 3, there is illustrated the internal structureofone of the pressure filter units 28. The pressure filter unit iscomprised of a tubular outer housing or casing 41 which has an elbow 42at its lower end connected through a suitable quick release coupling 43to a conduit 44. The upper end of the outer casing 41 is connected to anelbow fitting 46 by another quick release coupling 47. The elbow fitting46 in turn is connected by a further quick release coupling 48 to aconduit 49. The couplings 43, 47 and 48 are of a suitable conventionalcommercially available type and, preferably, are of the cam-lockingquick release type, in order that the filter unit parts can be quicklyseparated from each other for repair or replacement purposes.

A filter element 51 is disposed within the tubular outer casing 41. Thefilter element 51 is comprised of a perforate filter tube 52 which isclosed at its bottom end by an imperforate plate 53. The tube 52 has anannular flange 54 at its upper end. The radially outer portion of theflange 54 rests on a shoulder 56 provided adjacent the upper end of theouter casing 41. The lower end of the fitting 46 is adapted to clamp theflange 54 in position on the casing 41. The upper end of the tube 52 isopen and it communicates directly with the internal passage of thefitting 46.

If desired, a diffuser structure 57 may be disposed within the tube 52as set forth in U.S. Pat. No. 3 387 712 to ensure effective and uniformcleaning of the filter element 51 during the backwash cycle.

Likewise if desired, an air-operated vibrator assembly such as thatshown in U.S. Pat. No. 3 161 591 may be disposed within the outer casingand outside the filter element. Such vibrating unit transmits a highfrequency, low amplitude vibration to the process liquid in the regionof the filter area and ensures a high flow and positive clarificationwhile maintaining a uniform condition of the slurry or process liquidbeing filtered.

The filter tube 52 may be constructed in a wide variety of known ways.For example, the filter tube 52 can be made of a suitable reinforcedmetal screen or, preferably, the filter tube is comprised of a slottedmetal tube. The invention is not concerned with the details of theconstruction of the filter tube 52 and further description thereof isbelieved to be unnecessary.

The upper conduit 49 is connected to a three-way valve 71 so that theconduit 49 can be connected alternatively to the backwash inlet header33 or to the process liquid outlet header 30. Similarly, the lowerconduit 44 is connected to a threeway valve 72 so that said conduit canbe connected alternatively to the process liquid inlet header 29 or to aconduit 74. The valves 71 and 72 are illustrated in FIG. 3 as being inthe position they occupy during the filtration cycle wherein the processliquid or slurry to be filtered flows from the inlet header 29 throughthe conduit 44, thence through the filter unit 28 and the upper conduit49 to the outlet header 30. When the valves 71 and 72 are rotatedcounterclockwise through an angle of 90 from the positions shown in FIG.3, they will then be positioned for carrying out the recovery andbackwash cycles as will be described hereinbelow. During the recoveryand backwash cycles, the process liquid inlet header 29 and outletheader 30 do not communicate with the filter unit 28.

Referring now to the structure associated with the filter unit 28 forcarrying out the recovery and backwash cycles, the conduit 32 has atwo-way valve 76 therein for controlling the supply of backwash liquidto the backwash header 33. It will be understood that the valve 76 willbe in the closed position as shown in FIG. 3 except during the backwashcycle. The valve 76 will be rotated 90 from the position shown in FIG. 3to its open position when it is desired to backwash one or more of thefilter units 28. The backwash header 33 is common to all of the filterunits 28 and itisconnected to the respective filter units by means ofconduits 77. An air breaker valve 78 is attached to each of the conduits77 or, alternatively, a single air breaker valve can be mounted on thebackwash-inlet header 33. The air breaker valve 78 is operated after thebackwash cycle for a filter unit is completed so as to place the filterunit in communication with the atmosphere whereby to permit gravitydraining of the backwash liquid from the filter unit, prior toreintroducing the process liquid to be filtered during the nextfiltration cycle. By completely draining the backwash liquid from thefilter unit prior to starting the next filtration cycle, dilution of theincoming process liquid for the next filtration cycle is avoided.

Turning now to the modifications in the prior art system embodying thepresent invention, the conduit 77 has a compressed gas, normallycompressed air, supply line 79 connected thereto. A valve 81 is providedin the air supply line 79 and said line is connected to an air supplyheader 82 which is common to all the filter units. The valve 81 isclosed except during the recovery cycle. During the recovery cycle, thevalve 81 is opened, the valve 76 is closed and the valve 71 is rotated90 counterclockwise from the position shown in FIG. 3 wherebypressurized gas flows through the conduits 77 and 49 and thencedownwardly through the filter unit 28 in order to displace or purge fromthe filter unit all process liquid remaining therein at the end of thefiltration cycle and prior to beginning the backwash cycle.

The connection of the compressed air line 79- to the conduit 77 isadvantageous because it insures that the flow path for the processliquid during the filtration cycle does not communicate with the airline 79, so that an erroneous opening of the valve 81 during thefiltration cycle will not cause air to become present in the processliquid. Also, if the airline 79 were connected to the fitting 46 orconduit 49, it is possible that solid particles might collect oragglomerate thereat and that such particles could be discharged into theprocess liquid, after it has been filtered. This would cause a defect inthe coating on the paper web. Thus, by locating the air line 79 outsidethe process fluid flow path during the filtration cycle, improvedresults are achieved.

The process liquid discharged from the filter unit 28 by the compressedair form line 79 flows to the recovery and reconditioning system as willbe described hereinbelow. Positive pressure purging of the residualprocess liquid in the filter unit at the end of the filtration cycle isnecessary because of the high viscosity of the process liquid. Gas,normally air, is used for purging purposes in order to avoid dilution ofthe residual process liquid sent to the recovery and reconditioningsystem.

Both the recovered process liquid and the backwash liquid aredischarged, at different times, from the filter unit 28 through theconduit 74. The conduit 74 has a three-way valve 83 therein foralternatively directing fluid into either the backwash outlet header 34or into the recovery system conduit84. The conduit 84 is kept as shortas possible and is arranged either horizontally or downwardly inclinedso as to facilitate flow of the process liquid to be recovered. Theconduits 84 from all the pressure filter units 28 are connected to acommon recovery header 86.

The valve 83 is illustrated in FIG. 3 in the position it occupies duringthe backwash cycle in which conduit 74 communicates with the backwashoutlet header 34. During the recovery cycle, the valve 83 is rotateddegrees clockwise from the position illustrated in FIG. 3 in order toplace conduit 74 in communication with the conduit 84. The position ofthe valve 83 during the filtration cycle is not important to the basicfunction of the apparatus because at'that time it is isolated from thesystem by the valve 72, although for convenience in automating thecontrol thereof it is preferable to have valve 83 positioned in thefiltration step to connect conduit 74 with salvage conduit 84..

Referring to FIG. 2, there is illustrated a filtering assemblyconsisting of three pressure filter units 28 each having the associatedconduits and valves as described above. It will be understood that thenumber of filter units 28 in a particular installation is variabledepending on the requirements of the particular installation involved.For example, a single pressure filter unit can be used whereintermittent operation is satisfactory and multiple pressure filterunits can be used where a continuous operation is to be carried out. Ina multiple filtering unit installation as illustrated in FIG. 2, eachfilter unit 28 operates independently of the others and it can be takenout of the system by operation of its as sociated valves, withoutaffecting the operation of the other filter units. Thus, a backwashcycle for one unit can be carried out while the other filter units arestill in their filtration cycles. Further, each filter unit can be 7quickly and easily physically removed for servicing or repair bymanipulation of its associated couplings, without affecting theoperation of the other filter units.

The valves 71 and 72 for each filter unit have actuating levers 91 whichare coupled together, here by a rod 92, whereby both valves may beactuated simultaneously at the beginning and at the end of thefiltration cycle. An actuating handle 93 is removably receivable in asocket in the lower levers 91 to effect actuation of the valves 71 and72 for each filter unit. The valves 76, 81 and 83 are illustrated asbeing manually operated valves, but it will be understood that thesevalves could be operated by power means, eg fluid power or electricpower, and, further, it will be understood that the entire'cycle ,ofoperation of the filter apparatus could be automated by utilizing asuitable master control.

Referring to FIG. 4, there is illustrated an apparatus for treating theresidual process liquid removed from the filter units during therecovery cycle so that it can be returned to the process liquidutilization system for reuse. In this embodiment, the recovery header 86is connected by a conduit 96 to a filter apparatus 97. The filterapparatus 97 is preferably of the gravity type and it comprises a vessel98 having a filter medium 99, such as an inclined screen, extendingacross the vessel between the top and the bottom thereof so that therecovered process liquid drains by gravity through the filter medium 99.It will be understood that the filter apparatus 97 is not alwaysessential and, further, that a variety of different filter arrangementscould be used at this position, such as one employing an oscillatingfilte medium.

Referring to FIG. 5, there is illustrated a preferred arrangement of theconduits 84, recovery header 86 and conduit 96. It will be noted thatconduit 96 is positioned at one end of the header 86. Further, it willbe preferred to conduct the recovery and backwash cycles in the sequenceof filter units I, II and III. Thus, during the recovery cycle of filterunit III the entirety of the recovery header 86 will be cleaned out soas to remove therefrom any liquid which remains present therein from therecovery operations of the previously cycled filter units I and II.Thus, the prolonged presence of process liquid in the recovery system isavoided. This is desirable particularly if the process liquid is highlyviscous, in order to prevent settling of solids in the recovery system.

Referring again to FIG. 4, the vessel 98 has a conduit 101 extendingdownwardly from the bottom wall thereof. A three-way valve 102 isprovided in the conduit 101 whereby the filtered recovered processliquid can be fed into a tank 103 or, alternatively, it can be directedthrough the line 104 and then to drain. During normal recoveryoperations, the valve 102 will be positioned as illustrated in FIG. 4 todirect the recovered process liquid into the tank 103, from whence itcan be returned to the process liquid utilization system, for ex ample,by flowing it into the conduit 22.

In some instances, particularly when the process liquid has a highsolids content, it may be advisable to direct a quantity of the backwashliquid through the recovery system, from time to time, for the purposeof purging same of any process liquid present therein. It will normallybe suitable to purge the recovery system at the end of the backwashcycle for the last pressure filter unit III in the particularinstallation involved. However this can be done more frequently, ifdesired, such as at the end of the backwash cycle of each pressurefilter unit.

During the recovery system purging cycle, the valve 102 is rotatedclockwise from the position shown in FIG. 4 to discharge the liquid intothe line 104 whereby it flows to drain. Discharge of the liquid duringthe recovery system purging cycle to drain is advisable because thisprocess liquid will be diluted and ordinarily will be unsuitable forreturn to the process liquid utilization system.

MODIFICATION FIG. 6 illustrates a modified recovery and reconditioningsystem which is essentially similar to the system- OPERATION Althoughthe operation of the filtration system has been indicated above, thesame will be reviewed in sequence in order to insure a clearunderstanding of the invention. A complete cycle of operation of onlyone filter unit will be described and it will be understood that thesame sequence of operations will take place in all filter units. Thecycling of the respective filter unit can take place at the same time orat different times in accordance with any conventional desirableoperating procedure.

A. Filtration Cycle During the filtration cycle of any individual filterunit 28, the valves 71 and 72 are positioned as shown in FIG. 3. Thus,the process liquid to be filtered flows from the process liquid inletheader 29 to the conduit 44, thence through the filter unit 28 and theupper conduit 49 to the process liquid outlet header 30. The filtrate istherefore supplied in a clarified condition to the process liquidutilization system, such as to the pan 13 in FIG. 1. This operation isconventional and is not believed to require further detaileddescription.

B. Recovery Cycle At the end of the filtration cycle and prior tobeginning the backwash cycle, the valve 83 is in a position such thatconduit 74 is in communication with conduit 84. Then, the valves 71 and72 are rotated 90 counterclockwise as appearing in FIG. 3 to disconnectthe process liquid inlet header 29 and outlet header 30 from theconduits 44 and 49. At this time, there is present in the filter unit 28a quantity of the process liquid undergoing filtration.

This includes, for example, a quantity of process liquid outside thetube 52 and a quantity of filtrate in side said tube. It is desired torecover the retained process liquid in a condition suitable for returnto the utilization system. Possibly some flow of the retained processliquid into the recovery system may occur due After the valves 71 and 72have been rotated 90' counterclockwise as above described, the valve 81is opened to direct pressurized gas, usually air, into the conduit 77from whence it flows into the conduit 49, thence downwardly through thefilter unit 28 whereby the process liquid retained in the filter unit isdisplaced through the conduit 44, valve 72, conduit 74, valve 83 andconduit 84 into the recovery header 86.

The use of an air-assisted discharge of the retained process liquid fromthe filter unit 28 insures that the process liquid, even though it is atleast somewhat viscous, will be substantially completely discharged fromthe filter unit 28 into the recovery header 86. The recovered processliquid will then flow through the conduit 96 into the filter apparatus97 and thence through the conduit 101 and valve 102 into the tank 103.Some of the filter cake on filter medium 52 may be loosened and flowinto the recovery system. This is removed from the recovered processliquid by the screen 98.

During the recovery cycle, the valve 102 is positioned as shown in FIG.4. Also, the valve 76 is closed as shown in FIG. 3. At the end of therecovery cycle, the air supply valve 81 is closed and the valve 83 isrotated 90 clockwise back to the position shown in FIG. 3.

C. Backwash Cycle Then the valve 76 is opened to supply backwash liquidto the backwash header 33. The backwash liquid flows from the header 33through the conduit 77, valve 71, conduit 49, filter unit 28, conduit44, valve 72, conduit 74 and valve 83 to the backwash outlet header 34.The flow of the backwash liquid will flush clean the filter element 52and the filter cake particles will be discharged into the backwashoutlet header 34. Since the process liquid trapped in the filter unit atthe end of the filtration cycle was previously removed during therecovery cycle, the amount of the process liquid flowing to drain duringthe backwash cycle will be negligible. It is usually satisfactory toflow backwash liquid through the filter unit for a period of about 10 toseconds at a flow rate of about 175 to 200 gallons per minute to effectthorough cleaning of the filter element. This operation again isconventional and is not believed to require further description.

If purging of the recovery system is not required, then the followingrecovery system purge cycle can be omitted. However, if purging of therecovery system is to be carried out, then the following steps will betaken.

D. Recovery System Purge Cycle The recovery system purge cycle will notbe essential following each backwash cycle of each filter unit. Rather,it will ordinarily be sufficient to carry out a recovery system purgecycle only after the last one of a group of filter units in a filtrationapparatus has been backwashed. However, if desired or necessary becauseof the requirements of a particular installation, a recovery systempurge cycle can be carried out following'each backwash cycle of eachpressure filter unit.

In order to clean out the recovery system, after the backwash cycle iscompleted, the valve 83 is again rotated 90 clockwise from the positionshown in FIG. 3 and the valve 102 (or 102A) is rotated 90 clockwise asappearing in FIG. 4. Thus, the backwash liquid still flowing through thefilter unit 28 passes through the conduit 84, recovery header 86,conduit 96, filter 97 V and valve 102 and thence to drain to therebypurge any residual process liquid from the recovery system.Such apurging cycle requires only a few seconds and as soon as it iscompleted, the valves 83 and 102 are actuated to restore them to thepositions illustrated in FIGS. 3 and 4, respectively. It is understoodthat the purge water in the recovery system may be itself purgedforceably by air if desirable.

E. Filter Unit Drai'n Cycle Following completion of the backwash cycleand the recovery system purge cycle, if used, the valve 76 is closed andthen the air breaker valve 78 is opened, whereby the backwash liquid inthe conduit 49, filter unit 28, conduit 44 and conduit 74 drains bygravity into the backwash outlet header 34. As soon as this iscompleted, the valves 71 and 72 are then restored to their FIG. 3positions whereupon the foregoing described filtration cycle can berepeated. Since the filter unit 28 has been drained of backwash liquid,the

incoming process liquid for the next filtration cycle will not bediluted.

The operation of a filtration system employing a plurality of filterunits 28 will follow the above-described procedure, although ordinarilyonly one filter unit will undergo recovery, backwash, purge and draincycles at one time. The other filter units in a given filtration systemwill remain on stream so that the supply of filtered process liquid tothe process liquid utilization system will not be interrupted.

It will be observed that the additional components of the recoverysystem including conduit 79, valve 83, conduits 84, etc. are locatedoutside of the flow path of the process liquid during the filtrationcycle. Thus, the provision of these components will not interfere withthe smooth uninterrupted flow of the process liquid. Therefore, theprovision of the recovery system will not increase the possibility oflarge size particles becoming present in the process liquid,Particularly downstream of the filter medium 52. Further, because thecomponents of the recovery system are not connected to the pressurefilter units between the couplings 43 and 48, each filter unit 28 can beeasily removed from the system for servicing and repair and withoutinterfering with the recovery system.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A filtration system comprising a housing having a filter mediumdisposed in the interior thereof, a first inlet for supplying a processfluid to the housing on one side of said filter medium, a first outletfor receiving filt tered process fluid from the housing from theopposite side of said filter medium, a second inlet for supplying abackwashing fluid to the housing on said opposite side of said filtermedium, a second outlet for receiving the backwashing fluid from saidhousing from said one side of said filter medium;

valve means for alternatively placing (1) said first inlet and saidfirst outlet in communication with said housing, and (2) said secondinlet and said second outlet in communication with said housing, inorder to effect filtration and backwash cycles, respectively;

a recovery system associated with said second outlet for receivingprocess fluid present in said housing at the end of a filtration cycleand prior to a backwash cycle;

compressed gas supply means connected to said housing through a pathshared by said first outlet and said second inlet and energizable aftersaid first outlet is disconnected from said housing and prior toconnection of said second inlet to said housing for forceablydischarging residual process fluid from said housing.

2. A filtration system according to claim 1, wherein said first outletincludes an outlet conduit for said process fluid, said second inletincludes an inlet conduit for said backwash fluid, a discharge conduitleading from said housing, a valve connected between said dischargeconduit and said process fluid outlet conduit and said backwash fluidinlet conduit for alternatively connecting same to said dischargeconduit, said compressed gas supply means being connected to saidbackwash fluid inlet conduit and isolated from said process fluid outletconduit by said valve.

3. A filtration system according to claim 1, including an inlet conduitfor said process fluid, an outlet conduit for said backwash fluid and anentrance conduit leading from said housing, a valve connected betweensaid entrance conduit and said process fluid inlet conduit and saidbackwash fluid outlet conduit for alternatively connecting same to saidentrance conduit, and a further valve connected between said backwashfluid outlet conduit and said recovery system.

4. A filtration system according to claim 3, in which said recoverysystem contains a valve for alternatively directing fluid flowingtherethrough into a filter vessel for recycling the process fluid to thefiltration system or to discharge same from the filtration system.

5. A filtration system according to claim 1 including a plurality ofhousings each having a filter medium therein and each having a secondoutlet, said recovery system including a recovery header, a plurality ofvalves connecting corresponding ones of said second outlets to saidrecovery header, said recovery system further including a filter vesseland means connecting same to said recovery header for removing largeparticles from residual process fluid recovered from said plurality ofhousings prior to backwashing.

6. In a filtration system comprising a closed housing having a firstconduit at one end thereof, a second conduit at the opposite endthereof, a filter medium disposed in said housing between said endsthereof, a process liquid inlet and a backwash liquid outlet and firstvalve means for alternatively connecting said process liquid inlet andbackwash liquid outlet to said first conduit, a process liquid outletand a backwash liquid inlet and second valve means for alternativelyconnecting said process liquid outlet and said backwash liquid inlet tosaid second conduit,

the improvement comprising recovering means for forcibly removing andrecovering residual process liquid from said housing following afiltration step and prior to a backwashing step, so as to avoidcross-contamination of said residual process liquid and backwashingliquid, said recovering means including a pressure gas inlet connectedto said second conduit for pressurizing said housing and a recoverysystem connectible to said first conduit for receiving residual processliquid driven from said housing by said gas, said first valve meansincluding a first three-way valve connected between said process liquidinlet and said backwash liquid outlet for alternately connecting same tosaid first conduit, said backwash liquid outlet including a dischargeconduit adjacent said first valve, a second three-way valve in saiddischarge conduit and alternately connectible to said recovery systemand a backwash liquid receiver, said second valve means comprising athird three-way valve connected between said process liquid outlet andsaid backwash liquid inlet for alternately connecting same to saidsecond conduit, said pressure gas inlet comprising a compressed gassupply conduit including a valve and connected to said backwash liquidinlet, said gas supply conduit being separated from the outlet path ofsaid process liquid by said third three-way valve, and an air breakervalve on said backwash inlet conduit adjacent said gas supply conduitopenable at the end of a backwash cycle to place the filter unit incommunication with the atmosphere, whereby residual process liquidremaining in the housing after a filtration step is forcibly purged fromthe housing and recovered for future use before backwash liquid isintroduced into the housing. 7. In a filtration system'comprising aclosed housing 0 having a first conduit at one end thereof, a secondconduit at the opposite end thereof, a filter medium disposed in saidhousing between said ends thereof, a process liquid inlet and a backwashliquid outlet and first valve means for alternatively connecting saidprocess liquid inlet and backwash liquid outlet to said first conduit, aprocess liquid outlet and a backwash liquid inlet and second valve meansfor alternatively connecting said process liquid outlet and saidbackwash liquid inlet to said second conduit,

the improvement comprising recovering means for forcibly removing andrecovering residual process liquid from said housing following afiltration step and prior to a backwashing step, so as to avoidcross-contamination of said residual process liquid and backwashingliquid, said recovering means including a pressure gas inlet connectedto said second conduit for pressurizing said housing and a recoverysystem connectible to said first conduit for receiving residual processliquid driven from said housing by said gas, said second valve meanscomprising a three-way valve connected between said process liquidoutlet and said backwash liquid inlet for alternatively connecting sameto said second conduit, said pressure gas inlet comprising a valvedcompressed gas supply conduit connected to said back wash liquid inletand separated from the outlet path of said process liquid by saidthreeway valve, whereby residual process liquid remainalternativelyconnecting same to said first conduit, said backwash liquid outletincluding a discharge conduit adjacent said first valve, a thirdthree-way valve in said discharge conduit and alternately connectible tosaid recovery system and a backwash liquid receiver, said recoverysystem having a fourth three-way valve for alternatively directingresidual process liquid into a recycle vessel and to drain. 1

9. Improvement according to claim 8, in which the recovery systemincludes gravity filter means for removing large solids between saidthird and fourth three-way valves.

10. Improvement according to claim 8, in which the recovery systemincludes gravity filter means for removing large solids between saidfourth three-way valve and said recycle vessel.

11. Improvement according to claim 8, in which a series of closedhousings having filter mediums therein are connected in parallel, saidthird three-way valves for all of said housings being connected by shortconduits to' a common recovery header which communicates with a singlefourth three-way valve.

12. Improvement according to claim 7, in which said housing is tubularand has a tubular filter element therein, a vibrating device mounted insaid housing outside of said tubular filter element and releasablecouplings between said housing and said first and second conduits.

13. A filtering method comprising the steps of (1) during a filtrationcycle, flowing a process fluid to be filtered in a first directionthrough a housing having a filter medium therein in order to removeunwanted particles from the process fluid, (2) at the end of thefiltration cycle, terminating flow of the process fluid by adjusting afirst valve to disconnect the inlet of the housing from the supplyconduit for the process fluid and connecting said inlet to a drainsystem, leaving free residual process fluid in the housing, (3)injecting gas under pressure into said housing in the reverse directionand forcibly driving said free residual process fluid from said housingin said reverse direction so that the housing is empty of process fluidby adjusting a second valve to disconnect the outlet of the housing fromthe outlet conduit for the process fluid and connecting same to an inletconduit for said gas and backwashfluid and thenby admitting said gas tosaid inlet conduit (4) recovering said residual process fluid for reuseby discharging said residual process fluid retained in said housingthrough a further valve in said drain system to a recovery system whilepreventing said backwash fluid from entering said inlet conduit, (5)then flowing a backwash fluid through said housing in said reversedirection in order to remove contaminate from said filter element anddischarging the backwash liquid and contaminate particles entrainedtherein from the housing so that housing is empty of backwash fluid, and(6) then reintroducing process fluid into the housing for repeating thefiltration cycle whereby contact and cross-contamination between processand backwash fluids in and adjacent the housing is substantiallyeliminated.

14. A filtering method according to claim 13 in which the process fluidis a slurry and the step of recovering said residual process fluidincludes screenin s e to emove a c es ized lar er than sol'ds no rrn a ll y in e proc ss iluid tiierefrorr prior to reintroducing said residualprocess fluid into the filtering system.

15. A filtering method according to claim 13, includ ing the furtherstep of flowing backwash liquid through the recovery system at selectedintervals to purge process fluid therefrom and then discharging saidbackwash fluid from the recovery system.

16. A filtering method according to claim 13, including the step ofventing said housing to the atmosphere following backwashing whiledraining the housing of residual backwash fluid so as to remove allbackwash fluid from the housing.

17. A filtering method according to claim 16, in

which the filtered process fluid is a high viscosity slurry of highsolids content, the backwashing fluid is water and the gas is air.

2. A filtration system according to claim 1, wherein said first outletincludes an outlet conduit for said process fluid, said second inletincludes an inlet conduit for said backwash fluid, a discharge conduitleading from said housing, a valve connected between said dischargeconduit and said process fluid outlet conduit and said backwash fluidinlet conduit for alternatively connecting same to said dischargeconduit, said compressed gas supply means being connected to saidbackwash fluid inlet conduit and isolated from said process fluid outletconduit by said valve.
 3. A filtration system according to claim 1,including an inlet conduit for said process fluid, an outlet conduit forsaid backwash fluid and an entrance conduit leading from said housing, avalve connected between said entrance conduit and said process fluidinlet conduit and said backwash fluid outlet conduit for alternativelyconnecting same to said entrance conduiT, and a further valve connectedbetween said backwash fluid outlet conduit and said recovery system. 4.A filtration system according to claim 3, in which said recovery systemcontains a valve for alternatively directing fluid flowing therethroughinto a filter vessel for recycling the process fluid to the filtrationsystem or to discharge same from the filtration system.
 5. A filtrationsystem according to claim 1 including a plurality of housings eachhaving a filter medium therein and each having a second outlet, saidrecovery system including a recovery header, a plurality of valvesconnecting corresponding ones of said second outlets to said recoveryheader, said recovery system further including a filter vessel and meansconnecting same to said recovery header for removing large particlesfrom residual process fluid recovered from said plurality of housingsprior to backwashing.
 6. In a filtration system comprising a closedhousing having a first conduit at one end thereof, a second conduit atthe opposite end thereof, a filter medium disposed in said housingbetween said ends thereof, a process liquid inlet and a backwash liquidoutlet and first valve means for alternatively connecting said processliquid inlet and backwash liquid outlet to said first conduit, a processliquid outlet and a backwash liquid inlet and second valve means foralternatively connecting said process liquid outlet and said backwashliquid inlet to said second conduit, the improvement comprisingrecovering means for forcibly removing and recovering residual processliquid from said housing following a filtration step and prior to abackwashing step, so as to avoid cross-contamination of said residualprocess liquid and backwashing liquid, said recovering means including apressure gas inlet connected to said second conduit for pressurizingsaid housing and a recovery system connectible to said first conduit forreceiving residual process liquid driven from said housing by said gas,said first valve means including a first three-way valve connectedbetween said process liquid inlet and said backwash liquid outlet foralternately connecting same to said first conduit, said backwash liquidoutlet including a discharge conduit adjacent said first valve, a secondthree-way valve in said discharge conduit and alternately connectible tosaid recovery system and a backwash liquid receiver, said second valvemeans comprising a third three-way valve connected between said processliquid outlet and said backwash liquid inlet for alternately connectingsame to said second conduit, said pressure gas inlet comprising acompressed gas supply conduit including a valve and connected to saidbackwash liquid inlet, said gas supply conduit being separated from theoutlet path of said process liquid by said third three-way valve, and anair breaker valve on said backwash inlet conduit adjacent said gassupply conduit openable at the end of a backwash cycle to place thefilter unit in communication with the atmosphere, whereby residualprocess liquid remaining in the housing after a filtration step isforcibly purged from the housing and recovered for future use beforebackwash liquid is introduced into the housing.
 7. In a filtrationsystem comprising a closed housing having a first conduit at one endthereof, a second conduit at the opposite end thereof, a filter mediumdisposed in said housing between said ends thereof, a process liquidinlet and a backwash liquid outlet and first valve means foralternatively connecting said process liquid inlet and backwash liquidoutlet to said first conduit, a process liquid outlet and a backwashliquid inlet and second valve means for alternatively connecting saidprocess liquid outlet and said backwash liquid inlet to said secondconduit, the improvement comprising recovering means for forciblyremoving and recovering residual process liquid from said housingfollowing a filtration step and prior to a backwashing step, so as toavoid cross-contamination of said resIdual process liquid andbackwashing liquid, said recovering means including a pressure gas inletconnected to said second conduit for pressurizing said housing and arecovery system connectible to said first conduit for receiving residualprocess liquid driven from said housing by said gas, said second valvemeans comprising a three-way valve connected between said process liquidoutlet and said backwash liquid inlet for alternatively connecting sameto said second conduit, said pressure gas inlet comprising a valvedcompressed gas supply conduit connected to said back wash liquid inletand separated from the outlet path of said process liquid by saidthree-way valve, whereby residual process liquid remaining in thehousing after a filtration step is forcibly purged from the housing andrecovered for future use before backwash liquid is introduced into thehousing.
 8. Improvement in a filtration system according to claim 7, inwhich said first valve means includes a second three-way valve connectedbetween said process liquid inlet and said backwash liquid outlet foralternatively connecting same to said first conduit, said backwashliquid outlet including a discharge conduit adjacent said first valve, athird three-way valve in said discharge conduit and alternatelyconnectible to said recovery system and a backwash liquid receiver, saidrecovery system having a fourth three-way valve for alternativelydirecting residual process liquid into a recycle vessel and to drain. 9.Improvement according to claim 8, in which the recovery system includesgravity filter means for removing large solids between said third andfourth three-way valves.
 10. Improvement according to claim 8, in whichthe recovery system includes gravity filter means for removing largesolids between said fourth three-way valve and said recycle vessel. 11.Improvement according to claim 8, in which a series of closed housingshaving filter mediums therein are connected in parallel, said thirdthree-way valves for all of said housings being connected by shortconduits to a common recovery header which communicates with a singlefourth three-way valve.
 12. Improvement according to claim 7, in whichsaid housing is tubular and has a tubular filter element therein, avibrating device mounted in said housing outside of said tubular filterelement and releasable couplings between said housing and said first andsecond conduits.
 13. A filtering method comprising the steps of (1)during a filtration cycle, flowing a process fluid to be filtered in afirst direction through a housing having a filter medium therein inorder to remove unwanted particles from the process fluid, (2) at theend of the filtration cycle, terminating flow of the process fluid byadjusting a first valve to disconnect the inlet of the housing from thesupply conduit for the process fluid and connecting said inlet to adrain system, leaving free residual process fluid in the housing, (3)injecting gas under pressure into said housing in the reverse directionand forcibly driving said free residual process fluid from said housingin said reverse direction so that the housing is empty of process fluidby adjusting a second valve to disconnect the outlet of the housing fromthe outlet conduit for the process fluid and connecting same to an inletconduit for said gas and backwash fluid and then by admitting said gasto said inlet conduit (4) recovering said residual process fluid forreuse by discharging said residual process fluid retained in saidhousing through a further valve in said drain system to a recoverysystem while preventing said backwash fluid from entering said inletconduit, (5) then flowing a backwash fluid through said housing in saidreverse direction in order to remove contaminate from said filterelement and discharging the backwash liquid and contaminate particlesentrained therein from the housing so that housing is empty of backwashfluid, and (6) then reintroducing process fluid intO the housing forrepeating the filtration cycle whereby contact and cross-contaminationbetween process and backwash fluids in and adjacent the housing issubstantially eliminated.
 14. A filtering method according to claim 13in which the process fluid is a slurry and the step of recovering saidresidual process fluid includes screening same to remove particles sizedlarger than solids normally in the process fluid therefrom prior toreintroducing said residual process fluid into the filtering system. 15.A filtering method according to claim 13, including the further step offlowing backwash liquid through the recovery system at selectedintervals to purge process fluid therefrom and then discharging saidbackwash fluid from the recovery system.
 16. A filtering methodaccording to claim 13, including the step of venting said housing to theatmosphere following backwashing while draining the housing of residualbackwash fluid so as to remove all backwash fluid from the housing. 17.A filtering method according to claim 16, in which the filtered processfluid is a high viscosity slurry of high solids content, the backwashingfluid is water and the gas is air.